The present invention relates to a noise generator. More particularly, it relates to pseudo-random-bit sequence generators.
In the prior art, random noise signals have been provided by analog noise generators, for example noise diodes and resistors, to generate white Gaussian noise. Alternatively, low pass filtering of the output of a pseudo-random-bit sequence generator (PRSG) also produces noise approximating white Gaussian noise characteristics. Pseudo-random-bit sequence generators typically produce a binary signal having a bit pattern with a finite, though long, pseudo-random-bit repeat sequence.
An example of a structure for a PRSG includes a single shift register. The shift register includes a plurality of memory cells or bit registers. The input to the first bit-register is sequentially clocked through the shift register by an external clock signal. The first bit-register input signal is determined by the result of the logical operation on selected bit-register outputs of the shift register. PRSGs of the class just described have a finite repeat pseudo-random-bit sequence or pattern which is generally related to the size of the PRSG's shift register. The elapsed time for the generation of the finite repeat sequence, of course, depends on the PRSG's input clock signal frequency.
An example of a PRSG is illustrated and described in The Art of Electronics (Cambridge University Press, 1980, p437-446). As described therein, the repeat sequence of an m-bit shift register is 2.sup.m -1. For example, in a PRSG comprised of a 4-bit register, the pseudo random sequence or bit pattern repeats itself every 15 clock cycles. It is not uncommon for PRSGs to use a 17-bit register which repeats every 131,071 cycles.
Consequently, a shift register of large bit capacity has a long repeat sequence, and an even longer repeat sequence could be achieved by serially connecting many shift registers of large bit capacity. However, registers of very large capacity are generally very costly. Further, the desired approximation of the white-noise-like signal characteristics of noise generators utilizing very long PRSG repeat sequences deteriorates, i.e., resembles less than a true white noise distribution characteristic.